Bandgap reference circuits are well known in the analog IC arts for generating a reference voltage based on the bandgap potential inherent in semiconductor materials, generally approximately 1.2 Volts. As IC technology shrinks in size with advances in semiconductor process technology, device supply voltages must inevitably be reduced accordingly to avoid breakdown of the devices. For ICs used in portable electronics, minimal power consumption is also highly desirable. Significant effort is therefore devoted to development of low voltage and low power IC design. Bandgap reference circuits are widely used to provide an accurately known voltage as a fundamental reference for other analog circuit blocks and for generating a bias current or reference current. Since bandgap reference generators provide references for associated circuitry, it is generally desirable to provide bandgap circuits that turn on as early as possible and stay on as long as possible when a supply voltage is present. Thus, it is highly desirable that bandgap circuits operate at low voltage and consume little power.
One commonly used bandgap circuit is the Brokaw cell. A simplified schematic of a Brokaw cell familiar in the arts is illustrated in FIG. 1. The Brokaw cell has an output voltage VBG described by the equation,VBG=Vbe+VT×ln(N)×(2×R2/R1)  [Equation 1].The Brokaw cell is relatively simple and accurate but its usefulness in low voltage applications is limited by its minimum supply voltage requirement,VDD>(VBG−Vbe+Vce+Vgs)  [Equation 2],where Vbe is the base-emitter voltage of the bipolar transistors, Vce is the minimum collector-emitter voltage for the linear region of operation for the bipolar transistors, and Vgs is the gate-to-source voltage drop across the PMOS transistors. Those familiar with the arts will recognize that for a typical analog process with MOS VT of 0.7V, the VDD level at which the Brokaw cell functions, referring to Equation 2, is limited to, VDD>(1.24V−0.7V+0.5V+0.8V)=1.84V. The dominant factor in reaching this supply level is that the base is biased at the bandgap voltage level of about 1.24V. Thus, the utility of the Brokaw cell is limited to applications where the minimum input voltage does not fall below the acceptable VDD, in this example 1.84V, substantially higher than the bandgap voltage in general. Also, it will be seen that the total quiescent current of the Brokaw cell shown in the example of FIG. 1 may be described by,Iq=2×Iptat+VBG/R3  [Equation 3].A lower quiescent current level is desirable in the arts in order to reduce power consumption.
An alternative bandgap circuit known in the arts is the IPTAT (current proportional to absolute temperature) circuit. A schematic of an IPTAT bandgap circuit known in the arts is depicted in FIG. 2. This type of circuit represents attempts to overcome the limited low voltage range of the Brokaw type circuit. The output bandgap voltage of the circuit of FIG. 2 may be described by,VBG=Vbe+VT×ln(N)×(R2/R1)+(Ib×R2)  [Equation 4].Comparison of Equation 4 with Equation 1 reveals that the error term (Ib×R2) may cause the IPTAT circuit to be less accurate than the Brokaw cell. The IPTAT circuit, however, operates at a lower voltage level as shown by,VDD>VBG+Vds  [equation 5].The total quiescent current of the example IPTAT circuit shown in FIG. 2 is,Iq=5×IPTAT  [equation 6].
Problems remain in the effort to obtain a bandgap circuit that is accurate, operable at low voltages, and efficient. Due to these and other challenges in implementing low voltage and low power bandgap circuitry, it would be useful and desirable in the arts to provide improved bandgap reference methods and circuits adaptable to various low voltage IC applications. Such methods and devices would be particularly advantageous due to their low voltage operating capabilities and for their capability for maintaining low power consumption, accuracy, and reduced manufacturing costs.